Steering system for vehicle

ABSTRACT

A motor driving method for a steering apparatus for applying an auxiliary steering force to a steering system with two motors. When the two motors are driven, a first one of the two motors is operated first. Thereafter, the other, second motor is driven, whereby smooth steering is effected.

FIELD OF THE INVENTION

This invention relates to a steering system for an automotive vehicleand, more particularly, to a motor driving method wherein a steeringassist force is applied to a steering system by means of two motorsprovided in the steering system.

BACKGROUND OF THE INVENTION

Vehicular steering systems include electric power steering apparatus andsteer-by-wire systems which are devoid of mechanical linkage.

An electric power steering apparatus is an auxiliary apparatus in which,when while driving a vehicle a driver turns a steering wheel, a motor isoperated to provide a supplementary steering force or a steering assistforce. In an electric power steering apparatus, a steering torque signaloutputted from a steering torque detecting part for detecting a steeringtorque arising in a steering shaft when the driver turns the steeringwheel, and a vehicle speed signal outputted from a vehicle speeddetecting part for detecting the speed of the vehicle, are used todrive-control an assisting motor for outputting an auxiliary steeringforce on the basis of control operation of a motor control part, toreduce the steering force that must be applied by the driver. The motorcontrol part sets a target current value for a motor current passingthrough the motor on the basis of the steering torque signal and thevehicle speed signal, obtains the difference between a signal pertainingto this target current value (a target current signal) and a motorcurrent signal fed back from a motor current detecting part fordetecting the motor current actually flowing through the motor, andperforms proportional/integral compensation processing (PI control) onthis difference signal to produce a signal for drive-controlling themotor.

Generally, electric power steering apparatus have been developed mainlyfor use in small vehicles; however, particularly in recent years, fromthe points of view of fuel economy and expansion of the scope of vehiclecontrol, the need has arisen for larger vehicles (such as passengervehicles from the 2000 cc class upward) to be provided with electricpower steering. In applying an electric power steering apparatus to alarge vehicle, because the weight of the vehicle is large, with aconstruction using a single motor, a large motor outputting a largeassisting force is required. Consequently, the size of the motor islarge, the freedom of layout with which it can be mounted to the vehicle(its mountability) deteriorates, furthermore a large, specialnon-standard motor and motor control/driving part therefor becomenecessary, and the cost of manufacture increases.

To overcome this, as constructions suited to electric power steeringapparatus for large vehicles like this, constructions which use twoassisting motors have been proposed (see, e.g., InternationalPublication No. WO99/29557, JP-A-2001-260908, JP-A-2001-151125).

However, when an electric power steering apparatus is constructed usingtwo motors, the following problems arise.

When two motors are used, a motor control/driving part is provided foreach motor. When two motor control/driving parts are provided in acontrol unit (ECU), there is a risk of some variation arising betweenthese motor control/driving parts. When such variation arises, adifference arises in the respective voltages applied to the two motors.As a result, variation arises in the motor characteristics, anddifferences arise in the assisting thrusts produced by the two motorsand in their operating speeds.

However, by providing a common gear mechanism for the output shafts ofthe two motors and connecting them to a rack shaft by this gearmechanism, the rack shaft can be actuated with these differences betweenthe two motors being absorbed. At this time, the difference between theproduced torques and speeds of the two motors are balanced by the outputof the motor with the higher output being reduced.

In this case, when the degree of the variation between the two motors ofthe combination is large, the consequent fall in the motor output islarge, and the problem arises of the motor output being insufficienteven when a preset motor control signal is applied to the motors fromthe control unit. In particular, when during vehicle travel the steeringwheel starts to be turned or is returning to its center position,deterioration of steering feel and deterioration of controllabilitycaused by motor output insufficiency is unavoidable. Because many largevehicles requiring large motor outputs are high-price vehicles, it isimportant that deficiency of steering feel caused by variation betweenthe two motors in the electric power steering apparatus is eliminated.

Also, there is a risk of it happening that in the neutral vicinity,where the direction of rotation of the motors changes, and when thesteering wheel is returning to its central position, due to variationsof the motor control/driving parts the rotation of one of the motorsmomentarily becomes opposite, and the assisting thrusts of the twomotors momentarily become opposed and cancel each other out, andresponsiveness deteriorates.

Accordingly, in steering apparatus such as electric power steeringapparatus having two motors, a motor driving method for a steeringapparatus which obtains a balance in the operation of the two motors andof the control/driving part for each motor and which improves steeringfeel and raises controllability and also provides good responsiveness atall times has been awaited.

Also, when two assisting motors are provided in an electric powersteering apparatus as described above, even when one of the motorsfails, by means of the other motor it is possible to reduce the manualsteering force required from the driver to below what it would be ifthere were no assisting force from a motor whatsoever. However, the sizeof the auxiliary steering force from the normal motor, which has notfailed, is the still same as it is when the two motors are bothoperating normally, and to perform the same steering as when the twomotors are operating normally the manual steering force load on thedriver is larger. When the vehicle is stationary and the steering loadis large, with one motor the burden on the driver may be great. Inparticular, in a heavy vehicle, because the thrust of the steeringgearbox is large, the burden on the driver is large.

Reference is now made to the chart of FIG. 22 showing a relationshipbetween the loads on the motors and the driver in a conventionalelectric power steering apparatus.

In an electric power steering apparatus, the ratio of the auxiliarysteering force provided by the motor and the steering force applied bythe driver is about 10:1. In particular, in the case of an electricpower steering apparatus having two motors, the ratio of the auxiliarysteering forces provided by the two motors to the steering force appliedby the driver is about 5:5:1.

When the two motors, motor A and motor B, are operating normally, theburden on the driver is small, as shown by the hatching in the graph onthe left side of the figure.

If motor A fails, because the auxiliary steering force becomes only thatprovided by motor B, the burden on the driver increases by the amount ofthe auxiliary steering force normally provided by motor A. That is, theload ratio of motor A to motor B to the driver becomes 0:5:6.Consequently, as shown by the case of motor A failure in the graph onthe right side of the figure, the burden on the driver includes theauxiliary steering force that should be provided by motor A. This isbecause the size of the auxiliary steering force provided by motor B isthe same as it is when motor A and motor B are both operating normally.

With respect to the normal motor, because a limit value of the currentto be passed through the motor is set in accordance with thecharacteristics and durability and so on of the motor, and a currentgreater than this current limit value cannot be passed through themotor, a larger auxiliary steering force cannot be provided, and theadditional load must be taken on by the driver.

When, in an electric power steering apparatus having a plurality ofmotors, at least one of the motors has failed like this, it is desirablefor the amount of auxiliary steering force allocated to the normal motorto be increased, to reduce the burden on the driver.

Also, when an electric power steering apparatus is constructed using twomotors, the kinds of problem described below arise; however, first,problems of electric power steering apparatus having a single motor willbe considered.

Normally, in an electric power steering apparatus having one motor, theoutput shaft of the motor is connected to the steering system by way ofa gear mechanism, which is a power transmission mechanism. This gearmechanism may have any of various forms. A typical one is the pinionassist type electric power steering apparatus, which has a speed reducerprovided on a pinion gear shaft and a motor with its output shaftconnected to this speed reducer. With a pinion assist type electricpower steering apparatus, in a rack-and-pinion gearbox made up of a rackshaft having a rack gear and a pinion gear driving this provided on asteering wheel shaft, a motor and a speed reducer are provided servingthe pinion gear. The pinion gear is driven by the motor via the speedreducer. By this means, steering force assistance of the steering systemcorresponding to the steering force is carried out.

In this electric power steering apparatus having a single motor, withthe gear mechanism made up of a pinion gear and a rack gear, if thetorque ripple of the motor is large, this torque ripple passes throughthe gear mechanism and manifests as vibration in the steering system andspoils the steering feel, and also the operating noise of the motorincreases. Consequently, the product quality of a vehicle equipped withthis electric power steering apparatus decreases.

In the case of a pinion assist electric power steering apparatus havingtwo motors as steering system assisting motors, because each of the twomotors is connected to the steering system by way of the rack-and-pinionmechanism described above and each of the two motors imparts a vibrationcaused by motor torque ripple to the steering system through this gearmechanism, the above-mentioned impairment of steering feel and motoroperation noise become still more marked. This is a problem which arisesgenerally in steering apparatus having two motors. Also, the problemarises with both brushless motors and motors with brushes.

Consequently, in steering apparatus such as electric power steeringapparatus having two motors on a steering system, there has been a needto suppress motor torque fluctuation caused by torque ripple arising ineach motor and thereby to reduce vibration in the steering system,improve steering feel, and raise controllability.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda motor driving method for a steering apparatus, which comprises a stepof providing two motors for applying an auxiliary steering force to asteering system; a step of, when the two motors are to be operated,operating one of the two motors first; and a step of, after the firstmotor is operated, operating the other motor.

In this motor driving method, for example in an electric power steeringapparatus, at the initial stage when the driver starts to turn thesteering wheel, that is, in the range where the auxiliary steering force(assisting thrust) from the steering force assisting motors is small,just one of the two motors is started, and steering is carried out withone motor only. Because in the low torque range at the start of steeringa sufficient steering force can be obtained with only one motor, the twomotors are not started simultaneously. With this construction, theoccurrence of torque fluctuation caused by time differences of controlcommencement arising as a result of the two motors being startedsimultaneously is eliminated. As a result, smooth control is madepossible.

Desirably, in the step of starting one of the motors first, the twomotors are used alternately. With this method, because the two motorsand the control/driving parts for the motors are operated at the startof turning alternately, the motors can be operated equal numbers oftimes.

Preferably, in the step of operating one of the motors first, the sameone of the two motors is operated. Specifically, for example when thesteering direction is one direction, one of the motors is operated firstand then the other is operated, and when the steering direction is theother direction, the other motor is operated first and then the firstmotor is operated. That is, by making the motor for starting turning thesame one motor at all times, a difference is established between thenumbers of times the two motors are operated, and the probability ofboth of the motors failing in quick succession is reduced.

Desirably, the output capacities of the two motors (motor sizes andoutput characteristics) are made different. With this method, forexample by changing the sizes of the two mounted assisting motors, it ispossible to carry out control optimized in correspondence with theobject of the control, such as to when torque is necessary and to whenresponsiveness is necessary.

Preferably, when only one of the two motors is operating, motor failuredetection is carried out. That is, by employing a motor driving methodwherein the start timing of the two motors is made different, adifference in the motor operating states is provided, and it becomespossible to detect which of the motors has failed from the steeringstate during steering.

According to a second aspect of the present invention, there is providedan electric power steering apparatus for use in a vehicle, whichcomprises a plurality of motors for applying a force in a direction inwhich a steering wheel is turned, and allocating means for allocating anauxiliary steering force among the motors, the allocating means havingcurrent allocation determining means for, when at least one of themotors has failed, increasing the allocation of auxiliary steering forceallocated to the normal motors.

In this electric power steering apparatus, when at least one of theplurality of motors has failed, because a current allocation determiningpart of an allocating part increases the allocation of auxiliarysteering force allocated to the normal motor or motors, the burden onthe driver is reduced.

Preferably, the auxiliary steering force corresponds to the motorcurrents allocated to the motors, and the allocating means also hascurrent limit value setting means for setting a current limit value foreach of the motors, and when at least one of the motors has failed, thecurrent limit value setting means sets the current limit value of thenormal motors to failure current limit values and the allocating meansallocates the motor currents in correspondence with the failure currentlimit values by way of the current allocation determining means, towhich the failure current limit values are inputted. That is, when atleast one of the motors has failed, a current limit value setting partsets the current limit values of the normal motors to failure currentlimit values, and in correspondence with the failure current limitvalues a current allocation determining part of the allocating partallocates motor currents greater than the normal current limit values soas to make maximal use of the capabilities of the normal motors, wherebythe auxiliary steering force is increased and the burden on the driveris reduced.

Desirably, timing means for detecting whether the state of the currentvalue of a normal motor being higher than its normal current limit valuehas continued for a predetermined time is provided, and, when it has,the current limit value setting means returns the current limit valuefrom the failure current limit value to the normal current limit value.That is, because, under the condition that the state of the currentvalue of a normal motor being greater than the normal current limitvalue has continued for a predetermined time, the current limit valuesetting part returns the current limit value from the failure currentlimit value to the normal current limit value, the normal motor can beused maximally in accordance with the durability of the normal motor,and the burden on the driver can be reduced.

According to a third aspect of the present invention, there is provideda steering apparatus wherein a driving force from a motor is transmittedthrough a gear mechanism to a rack shaft of a steering system and thedriving force is applied in the direction of steering of a steering roadwheels, having a first motor and a second motor and characterized inthat in the transmission of the driving forces from the respectiveoutput shafts of the first and second motors to the rack shaft the phaseof the two motors is essentially staggered by 180°.

With this steering apparatus, steering is carried out using two motorshaving the same capabilities, and by the electrical phases of the twomotors being staggered by 180°, even when torque fluctuations arise dueto torque ripple in the motors, at the rack shaft having the steeringroad wheels at its ends these torque fluctuations are opposite in phaseand cancel each other out, and overall motor torque fluctuation can besuppressed.

Preferably, the gear mechanism is a rack-and-pinion gear mechanism, andin the meshing relationship between two pinion gears connected to theoutput shafts of the two motors and rack gears corresponding to these,the phases of the pinion gears are essentially staggered by 180°. Bystaggering the phases of the pinion gears by 180° in this way, thetorque fluctuations transmitted from the pinion gears to the rack gearsare made opposite in phase, and mutual cancellation occurs and motortorque fluctuation can be suppressed.

In a preferred form, between the two motors, the waveform of the motortorque fluctuation arising from one of the motors and the waveform ofthe motor torque fluctuation arising from the other motor are set so asto be opposite in phase. That is, in the respective driving forcetransmission linkages of the two motors in the two-motor steeringapparatus, if setting is so carried out that the waveforms of the motortorque fluctuations arising in the transmission linkages are opposite inphase, the motor torque fluctuations can be suppressed.

Preferably, the two motors used in this steering apparatus of theinvention are used as electric power steering assisting motors forsupplementing a manual steering force. In an electric power steeringapparatus using two motors like this, if torque fluctuations caused bytorque ripple of the motors is suppressed, vibration in the steeringsystem of the electric power steering apparatus can be reduced and thesteering feel improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the present invention will now bedescribed in detail, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 is a construction view showing schematically the overallconstruction of an electric power steering apparatus to which a motordriving method according to the invention has been applied;

FIG. 2 is a view showing the external layout of an actual apparatus of arack shaft having two motors and a gear box;

FIG. 3 is a block diagram showing part of the internal construction of acontrol unit according to a first preferred embodiment of the invention;

FIG. 4 is an electrical function block diagram showing the internalconstruction of a motor control part shown in FIG. 3;

FIG. 5 is a flowchart showing a motor driving method of a first examplepertaining to the first preferred embodiment;

FIG. 6 is a flowchart showing a motor driving method of a second examplepertaining to the first preferred embodiment;

FIG. 7 is a flowchart showing a motor driving method of a third examplepertaining to the first preferred embodiment;

FIG. 8 is a flowchart showing a motor driving method of a fourth examplepertaining to the first preferred embodiment;

FIG. 9 is a flowchart showing a motor driving method of a fifth examplepertaining to the first preferred embodiment;

FIG. 10 is an electrical function block diagram showing the internalconstruction of a control unit according to a second preferredembodiment of the invention;

FIG. 11 is an electrical function block diagram showing a specificexample of a target current allocating part shown in FIG. 10;

FIG. 12 is a flowchart showing the operation of the target currentallocating part of the second preferred embodiment shown in FIG. 11;

FIG. 13 is a view showing a relationship of loads on motors and a driverpertaining to the second preferred embodiment;

FIG. 14 is an electrical function block diagram showing the internalconstruction of a control unit according to a third preferred embodimentof the invention;

FIG. 15 is an electrical function block diagram showing a specificexample of a target current allocating part pertaining to the thirdpreferred embodiment shown in FIG. 14;

FIG. 16 is a flowchart showing the operation of the target currentallocating part pertaining to the third preferred embodiment shown inFIG. 15;

FIG. 17 is a view showing a relationship of loads on motors and a driverpertaining to the third preferred embodiment;

FIG. 18 is an enlarged sectional view on the line 18—18 in FIG. 1,showing the internal structure of a first gearbox;

FIG. 19 is a sectional view on the line 19—19 in FIG. 18;

FIG. 20 is a view showing a relationship of two rack-and-pinionmechanisms at a rack shaft;

FIGS. 21A through 21C are views showing changes of motor torquefluctuations produced by the two rack-and-pinion mechanisms shown inFIG. 20, FIGS. 21A and 21B showing torque fluctuations of the twomotors, and FIG. 21C showing a torque fluctuation obtained by combiningthe torque fluctuations of the two motors; and

FIG. 22 is a view showing a relationship of loads on motors and a driverin an electric power steering apparatus of related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 and FIG. 2, an electric power steering apparatus 10is mounted in a passenger vehicle or a large passenger vehicle. Theelectric power steering apparatus 10 is constructed to apply anassisting steering torque to a steering shaft 12 connected to a steeringwheel 11. The upper end of the steering shaft 12 is connected to thesteering wheel 11, and a first pinion gear (or pinion) 13 a is attachedto the lower end.

Part of the lower end of the steering shaft 12 to which the first piniongear 13 a is attached will be called the first pinion shaft 12 a. Inpractice, the upper side of the steering shaft 12 and the first pinionshaft 12 a on the lower side are connected by a universal joint notshown in the figure.

A rack shaft 14 has two rack gears, a first rack gear 14 a and a secondrack gear 14 b, formed apart from each other in the length direction ofthe shaft.

The first pinion gear 13 a and the first rack gear 14 a form a firstrack-and-pinion mechanism 15A.

The first rack-and-pinion mechanism 15A, a first power transmissionmechanism 18A and a steering torque detecting part 20 are housed in afirst gearbox 24A. The exterior of the first gearbox 24A is shown inFIG. 2.

The rack shaft 14 has tie rods 16, 16 at its ends. Front wheels 17, 17functioning as steering road wheels of the vehicle are attached to outerends of the tie rods 16, 16.

The electric power steering apparatus 10 of this preferred embodimenthas two motors, a first motor 19A and a second motor 19B. The firstmotor 19A is connected by way of the first power transmission mechanism18A to the first pinion shaft 12 a. The second motor 19B is connected byway of a second power transmission mechanism 18B to a second pinionshaft 12 b. These first and second motors 19A, 19B are each operatedindependently with design details (output capacity and so on), properstart timing, and operating details determined independently for each,as will be further discussed later, and output turning forces (torques)for supplementing a steering torque, and these turning forces areapplied by way of the first and second power transmission mechanisms18A, 18B to the first and second pinion shafts 12 a, 12 b.

The steering torque detecting part 20 is provided on the steering shaft12. When a steering torque created by the driver turning the steeringwheel 11 is applied to the steering shaft 12, this steering torquedetecting part 20 detects the steering torque acting on the steeringshaft 12.

The reference number 21 denotes a vehicle speed detecting part fordetecting the speed of the vehicle. A control unit (ECU) 22 is made upof a computer system using a microcomputer.

The control unit 22 takes in a steering torque signal T outputted fromthe steering torque detecting part 20 and a vehicle speed signal Voutputted from the vehicle speed detecting part 21, and, on the basis ofinformation pertaining to the steering torque and information pertainingto the vehicle speed, outputs drive control signals SG1A, SG1B forcontrolling the operation of the first and second motors 19A, 19B. Thefirst and second motors 19A, 19B have first and second motor angledetecting parts 23A, 23B. Signals SG2A, SG2B pertaining to the angularpositions (electrical angles) of the first and second motor angledetecting parts 23A, 23B are inputted to the control unit 22.

The first motor 19A and the second motor 19B used in this embodimenthave identical constructions and capabilities.

As shown in FIG. 1 and FIG. 2, a second gearbox 24B houses the secondrack gear 14 b formed on the rack shaft 14; a second pinion gear 13 bmeshing with this second rack gear 14 b; the second pinion shaft 12 b,which is rotatably supported and to which the second pinion gear 13 b isattached; and the second power transmission mechanism 18B. The outputshaft of the second motor 19B has a transmission shaft (worm shaft).This transmission shaft has a worm gear. A worm wheel meshing with thisworm gear is fixed to the second pinion shaft 12 b. Thus the secondgearbox 24B has basically the same construction as the first gearbox24A.

When the second motor 19B is driven, driving force is transmittedthrough the output shaft, the worm gear, the worm wheel, the secondpinion shaft 12 b, the second pinion gear 13 b and the second rack gear14 b to the rack shaft 14.

As described above, the electric power steering apparatus 10 of thispreferred embodiment has a steering torque detecting part 20, a vehiclespeed detecting part 21, a control unit 22, two motors 19A, 19B, and twopower transmission mechanisms 18A, 18B.

In this construction, when the driver turns the steering wheel 11 tosteer the car while driving, a turning force based on the steeringtorque applied to the steering shaft 12 is converted through the firstpinion shaft 12 a at the bottom of the steering shaft 12 and the firstrack-and-pinion mechanism 15A to linear motion in the axial direction ofthe rack shaft 14, and then through the tie rods 16, 16 it changes thetravel direction of the front wheels 17, 17.

At this time, the steering torque detecting part 20 provided on thesteering shaft 12 simultaneously converts a steering torquecorresponding to the steering force applied to the steering wheel 11 bythe driver into an electrical steering torque signal T, and outputs thissteering torque signal T to the control unit 22. The vehicle speeddetecting part 21 detects the speed of the vehicle and converts this toa vehicle speed signal V, and outputs this vehicle speed signal V to thecontrol unit 22. The control unit 22 generates motor currents fordriving the two motors 19A, 19B on the basis of the steering torquesignal T and the vehicle speed signal V. The first and second motors19A, 19B driven by these motor currents apply auxiliary steering torquesto the rack shaft 14 by way of the first and second power transmissionmechanisms 18A, 18B. As described above, by the two motors 19A, 19Bbeing operated on the basis of a predetermined motor driving method, thesteering force applied to the steering wheel 11 by the driver isreduced.

As shown in FIG. 3, the control unit 22 has a first motor control part30A and a second motor control part 30B provided in parallel, fordrive-controlling the first motor 19A and the second motor 19B on thebasis of the steering torque information outputted from the steeringtorque detecting part 20. The control unit 22 also has a motor driveorder control part 33 for controlling the drive order of the two drivemotors 19A, 19B, and a storing device 34 storing a program for executingthis drive sequencing of the two motors.

The motor drive order control part 33 is a higher-level control partthan the first and second first motor control parts 30A, 30B, and isconstructed around a CPU.

The first motor control part 30A and the second motor control part 30Bhave essentially the same construction and function, and respectivelyhave target current determining parts 31A, 31B and control parts 32A,32B. The target current determining parts 31A, 31B determine targetauxiliary torques mainly on the basis of the steering torque signal T,and output signals (target current signals) IT pertaining to targetcurrent values necessary for supplying these target auxiliary torquesfrom the first and second motors 19A, 19B. The T-IT conversioncharacteristic shown in FIG. 3 is an example.

Next, a specific example of the control part 32A in the first motorcontrol part 30A will be described, on the basis of FIG. 4. Here,because the second motor control part 30B is the same as the first motorcontrol part 30A, a description of the control part 32B will be omitted.

As shown in FIG. 4, the control part 32A is made up of a differentialoperator part 41, a motor drive control part 42, a motor driving part 43and a current value detecting part 44.

The motor drive control part 42 is made up of a differential currentcontrol part 45 and a PWM signal generating part 46.

The differential current control part 45 generates and outputs a drivingcurrent signal for controlling the motor current on the basis of thecurrent signal from the differential operator part 41.

The PWM signal generating part 46 generates a PWM (Pulse WidthModulation) signal for PWM-driving the first motor 19A on the basis ofthe driving current signal from the differential current control part45.

The motor driving part 43 is made up of a gate driving circuit part 47and a motor driving circuit (an H-type bridge circuit formed with fourFETs) 48. The gate driving circuit part 47 switches the motor drivingcircuit 48 on the basis of the drive-control signal (PWM signal).

By this means, the first motor control part 30A shown in FIG. 3PWM-controls the motor current supplied to the first motor 19A from abattery power source 49 on the basis of the steering torque T detectedby the steering torque detecting part 20, and controls the poweroutputted by the first motor 19A (the auxiliary steering torque).

The following control is carried out in connection with the order ofoperation of the two motors 19A, 19B controlled by the control unit 22having this construction. This operation order control is carried out bythe motor drive order control part 33, which is the high-level controlpart shown in FIG. 3.

First, a motor driving method of a first example pertaining to the firstpreferred embodiment will be described, with reference to FIG. 5. Thismotor driving method is applied in the range where the assisting thrustof the electric power steering apparatus 10 is small (start of steering;low torque range), when the steering wheel 11 is starting to turn. Inthis motor driving method, at the start of steering, steering forcesupplementation is carried out with only one of the two motors 19A, 19B.And with the motor driving method of this first example, the ratings andoutput characteristics of the two motors 19A, 19B are the same.

In FIG. 5, when the driver operates the steering wheel 11 shown in FIG.1 to start turning or to return it, a signal pertaining to the steeringtorque of this steering state detected by the steering torque detectingpart 20 is inputted to the control unit 22. In the control unit 22, thedetection signal from the steering torque detecting part 20 is inputted(step S11), and it is determined whether or not there has been anapplication of steering torque or a reversal of steering torquecorresponding to this steering state (step S12). When in step S12 thedetermination is NO, steps S11, S12 are repeated. When in step S12 thedetermination is YES, the following step S13 is executed.

In step S13, one of the two motors 19A, 19B (for example the first motor19A) is started, and an assisting thrust is outputted.

In step S14, until it is determined that the assisting thrust from themotor 19A has reached its rated torque, the operation of the first motor19A is continued. As the signal for determining whether or not the ratedtorque has been reached, the above-mentioned steering torque signal Tmay be used, or alternatively a torque signal detectable from the firstmotor 19A can be used. Steps S13 and S14 are repeated until the ratedtorque is reached. When in step S14 it is determined that the assistingthrust from the first motor 19A has reached the rated torque, processingproceeds to step S15.

In step S15, of the two motors 19A, 19B, the operation of the motoralready operating is continued, and the other motor (in this case, thesecond motor 19B) is started and an assisting thrust is outputted fromthat. Accordingly, when step S15 is executed, assisting thrustsoutputted by the two motors 19A, 19B are applied to the steering system.

With this first motor driving method, because in the low torque rangeonly one of the two motors 19A, 19B is operated to provide steeringforce supplementation, torque fluctuation of a control commencement timedifference resulting from the two motors being started at the same timedoes not arise, and smooth assisting can be effected at the start ofturning or returning. And after the rated torque has been reached, thetwo motors 19A, 19B are operated simultaneously to output a largeassisting thrust.

Also, because the start timings of the two motors 19A, 19B are staggeredin correspondence with the assisting thrust required, the output dropcaused by torque interference of when the variation between the twomotors is large can be eliminated, and control which is stable withrespect to output can be carried out. In particular, at low assistingthrusts, such as at the start of turning or returning during travel,when torque fluctuations can be felt easily, control having littletorque fluctuation can be carried out.

Although in the motor driving method of the first example describedabove it was determined in step S14 whether or not a rated torque hadbeen reached, instead of determining on the basis of torque it ispossible alternatively to provide a step of determining whether or not arated duty has been reached. With this method, the determination can bemade with reference to the state of duty of the above-mentioned PWMsignal determining the motor current supplied to the motor. In this waythe prescribed value in step S14 can be changed.

Next, a motor driving method of a second example will be described, onthe basis of FIG. 6. In this motor driving method, the two motors 19A,19B are used alternately as the motor used at the start of steering whenthe steering wheel 11 starts to be turned. By this means, thefrequencies of operation of the two motors 19A, 19B can be made thesame. In the motor driving method of this second example, the ratingsand output characteristics of the two motors 19A, 19B are the same.

The control procedure of the motor driving shown in FIG. 6, from thepoint of view of the motor driving method as a whole, is insertedbetween the steps S12 and S14 shown in FIG. 5. When a predeterminedsteering torque state has been detected (step S12) and one of the twomotors 19A, 19B is to be started, steps S21 through S25 shown in FIG. 6are executed to determine which of the motors should be started first inthe step S13 shown in FIG. 5.

With this second motor driving method example, in step S21, with thepremise that an assisting thrust is to be produced, steeringcommencement processing is executed. After that, an operation startmotor history flag is checked (step S22). Here, the state of anoperation history flag of the two motors 19A, 19B is checked.

In the following step S23 the motor which was not used the previous timeis selected, and steering commencement of this motor is executed. As aresult, the step S13 shown in FIG. 5 is implemented. The subsequentsteps S24, S25 are post-processing steps. That is, under the conditionthat steering has ended (step S24), the history flag pertaining to themotor that was started first at the start of steering is updated (stepS25).

With the motor driving method of this second example, as the motorstarted at the start of steering, the two motors 19A, 19B are usedalternately, and the frequencies of operation of the two motors arethereby made equal. As a result, equal loads can be applied to the twomotors 19A, 19B and the control/driving parts of the motors, and theirlives can be equalized.

A motor driving method of a third example will now be described, withreference to the flowchart shown in FIG. 7. This motor driving method isa variation of the motor driving methods of the first and secondexamples. In this motor driving method, regarding the motor to be usedat the start of steering when the steering wheel 11 is starting to turn,one of the two motors 19A, 19B is allocated to rightward use only andthe other is allocated to leftward use only. Because rightward steeringand leftward steering occur in substantially the same proportions duringtravel, by this means the frequencies of operation of the two motors19A, 19B can be made equal. With this third motor driving methodexample, the ratings and output characteristics of the two motors 19A,19B are the same. In the control procedure of FIG. 7, steps the same assteps shown in FIG. 6 have been given the same reference numerals.

In the flowchart shown in FIG. 7, after the steering commencementprocessing step S21, a steering direction distinguishing step S31follows. When in step S31 the steering direction is determined to beleftward, the three steps S32, S33, S34 are carried out. These steps S32to S34 are essentially the same respectively as the steps S13 to S15shown in FIG. 4. When in step S31 the steering direction is determinedto be rightward, the three steps S35, S36 and S37 are executed. Thesesteps S35 to S37 are essentially the same respectively as the steps S13to S15 shown in FIG. 5. Finally, steering ending processing S25 iscarried out.

With this third motor driving method example, by the two motors 19A, 19Bbeing allocated to either rightward use or leftward use and each used atthe start of steering accordingly, the frequencies of operation of thetwo motors can be made equal and their lives made equal.

A motor driving method of a fourth example will now be described, on thebasis of the flowchart shown in FIG. 8. This motor driving method is avariation of the motor driving method of the first example. In thismotor driving method, the two motors 19A, 19B are designated as a mainmotor and a sub motor, and it is predetermined that the motor used atthe start of steering will always be the main motor. The ratings andoutput characteristics of the two motors may be the same or may bedifferent. In the flowchart shown in FIG. 8, steps essentially the sameas steps shown in FIG. 5 have been given the same reference numerals.

In FIG. 8, when the driver starts to turn or returns the steering wheel11, in the control unit 22, the detection signal from the steeringtorque detecting part 20 is inputted (step S11), and it is determinedwhether or not there has been an application of steering torque or areversal of steering torque corresponding to this steering state (stepS12). When in step S12 the determination is NO, steps S11, S12 arerepeated. When in step S12 the determination is YES, step S41 isexecuted.

In step S41, whichever of the two motors 19A, 19B is the main motor isstarted, and an assisting thrust is outputted. In the next step, stepS42, until it is determined that the assisting thrust from the mainmotor has reached a prescribed value (prescribed torque or prescribedduty), operation of the main motor is continued. That is, steps S41 andS42 are continued.

When in step S42 it is determined that the assisting thrust from themain motor has reached the prescribed value, processing proceeds to stepS43.

In step S43, while the operation of the main motor is continued, theremaining one of the two motors 19A, 19B, i.e. the sub motor, is startedand outputs an assisting thrust. Accordingly, when step S43 is executed,assisting thrusts are applied to the steering system by the two motors19A, 19B.

With the fourth motor driving method example described above, because inthe low torque range steering force supplementation is carried out withonly the main motor being operated, torque fluctuation of a controlcommencement time difference resulting from the two motors being startedat the same time does not arise, and smooth steering forcesupplementation can be effected at the start of turning.

Also, because the start timing of the two motors 19A, 19B is staggeredin correspondence with the assisting thrust required, the output dropcaused by torque interference of when the variation between the twomotors is large can be eliminated, and control which is stable withrespect to output can be carried out. In particular, at low assistingthrusts, such as at the start of turning or returning during travel,when torque fluctuations can be felt easily, control having littletorque fluctuation can be carried out.

Also, with this fourth motor driving method example, by using a mainmotor of the two motors at the start of steering, the total numbers oftimes that the two motors are driven can be made different, and adifference can be provided in the lives of the motors. Therefore, theprobability of both of the motors failing at the same time is reduced.And when one of the motors fails, by monitoring the value of theassisting torque it can be inferred which of the motors has failed. Forexample, when no torque is obtained at the start of steering it can beinferred that the main motor has failed, and when no torque is obtainedwhen an assisting torque is necessary it can be inferred that the submotor has failed.

Because it is possible to make the two motors 19A, 19B a main motor anda sub motor and differentiate the ratings and output characteristics ofthe motors in magnitude, it is possible to realize optimal control bothwhen torque is necessary and when responsiveness is necessary. Forexample, if a motor with a large output is used at the start of turning,when a smaller motor is then driven an ample torque is already present,and as a result the relative value of the torque fluctuation of when thesecond motor is started can be made small, and the steering feel can bemade good.

A motor driving method of a fifth example will now be described, withreference to the flowchart shown in FIG. 9. This motor driving methodshows an example of a control method for when one of the motors hasfailed. In this case, the ratings and output characteristics of the twomotors 19A, 19B may be the same or may be different.

Steps S51 to S56 shown in FIG. 9 correspond to processing for when oneof the motors is started at the start of steering. When the steeringwheel 11 is operated and steering force supplementation is carried outby the electric power steering apparatus 10, first, at the start ofsteering, one of the two motors is started and attempts to commencesteering (step S51). At this time, the output duty of the motor that wasstarted is determined (step S52).

When in step S52 the output duty of this motor is below a prescribedvalue, the operating state of this motor is continued without change.After that, as explained with reference to the flowchart of FIG. 5, itis determined whether or not a prescribed torque has been reached (stepS14), and processing for starting the other motor (step S15) is carriedout. When in step S52 the output duty of the first motor is larger thanthe prescribed value, the value of the motor current is determined (stepS53).

When in step S53 the value of the motor current is above a prescribedvalue, because the required motor output is being obtained, theoperating state of this first motor is continued without change. Afterthat, as explained on the basis of the flowchart shown in FIG. 5, it isdetermined whether or not a prescribed torque has been reached (stepS14), and processing for starting the other motor is carried out (stepS15).

When in step S53 the value of the motor current is smaller than theprescribed value, it is determined that the first motor has failed (stepS54), and the operation of this motor is stopped (step S55). After that,the other of the two motors is operated, and steering with this motor isstarted (step S56). In this case, steering force supplementation iscarried out with this second motor only.

With this fifth motor driving method example, when one of the two motors19A, 19B has failed, control is carried out with just the remainingmotor. Compared to a construction in which two motors are operatedsimultaneously, failure detection is easier, and failure detection canbe carried out in a short time.

In the motor driving methods of an electric power steering apparatushaving two motors described above, because steering forcesupplementation control is carried out with one motor only, controlwhich does not suffer an influence of reversing timing of the two motorscan be achieved.

In the first preferred embodiment described above, examples of motordriving methods for an electric power steering apparatus have beendescribed; however, a motor driving method for two motors provided in asteering system according to the invention can be applied to othersteering apparatus, such as steer-by-wire systems.

As described above, with this first preferred embodiment, because in thelow torque range a steering force is outputted with only one of twomotors provided in a steering system, torque fluctuation resulting fromcontrol commencement time difference is eliminated and smooth controlcan be carried out. And because control is started with one motor only,influences of reversing timing of two motors on return can be avoided.

Also, with this preferred embodiment, because the two motors can be usedalternately as the motor started first, the usage frequencies of the twomotors can be kept equal and their lives can be kept equal.

Also, with this preferred embodiment, because the first motor to startfirst is always the same, the usage frequencies of the two motor can bedifferent and the lives of the two motors can be different. Thus,failures of the two motors at the same time can be avoided.

Also, with this preferred embodiment, because two motors are providedand their starting timing is made different, failure detection of themotors is easy and can be effected rapidly.

Also, with this preferred embodiment, because the sizes of the twomotors provided can be made different, it is possible to effect optimalcontrol adaptable to different control objectives, such as that of whentorque is necessary and that of when responsiveness is necessary.

FIG. 10 is a block diagram showing the internal construction of acontrol unit 222 according to a second preferred embodiment of theinvention.

The control unit 222 is made up of a motor drive control part 243A forthe first motor 19A, a motor drive control part 243B for the secondmotor 19B, a target current setting part 231 for setting a targetcurrent on the basis of the steering torque signal T and the vehiclespeed signal V and outputting a target current signal IT, a failuredetecting part 242 for detecting failure of the motors 19A, 19B, and atarget current allocating part 250 for allocating target currents to themotors 19A, 19B.

The motor drive control part 243A is made up of a gate driving circuit247A and a motor driving circuit 248A. The motor drive control part 243Bis made up of a gate driving circuit 247B and a motor driving circuit248B. Power from a battery 249 is supplied to the motor driving circuits248A, 248B.

The failure detecting part 242 detects failures for example on the basisof the duties duty 1, duty 2 of the PWM signals applied respectively tothe motor driving signals 248A, 248B from the gate driving circuits247A, 247B.

The above-mentioned motor currents are currents passed through themotors 19A, 19B from the motor drive control parts 243A, 243B. The gatedriving circuit 247A of the motor drive control part 243A outputs a PWMsignal on the basis a current signal SA allocated to it by the targetcurrent allocating part 250, and switches the motor driving circuit 248Aon the basis of the duty of this PWM signal. By this means a motorcurrent is supplied to the first motor 19A. Also in the motor drivecontrol part 243B for the motor 19B, the gate driving circuit 247B andthe motor driving circuit 248B operate similarly on the basis of acurrent signal SB allocated by the target current allocating part 250.When it receives a failure signal K from the failure detecting part 242,the target current allocating part 250 identifies the failed motor andchanges the current signals SA, SB in accordance with the result so asto increase the motor current passing through the normal motor, whichhas not failed.

FIG. 11 is a block diagram showing the internal construction of thetarget current allocating part 250 shown in FIG. 10.

The target current allocating part 250 is made up of a failed motoridentifying part 251 for receiving the failure signal K and identifyinga failed motor and a current allocation determining part 252 forreceiving the target current signal IT and determining currents to beallocated to the two motors 19A, 19B and outputting the current signalsSA, SB. The failed motor identifying part 251 receives the failuresignal K and determines whether a failed motor is the first motor 19A orthe second motor 19B. The failed motor identifying part 251 outputs asignal KM pertaining to the normal motor to the current allocationdetermining part 252. The current allocation determining part 252receives the target current signal IT and the signal KM pertaining tothe normal motor and determines currents to be supplied to the firstmotor 19A and the second motor 19B. For example, when the first motor19A has failed, it makes the current to the first motor 19A zero andincreases the current to the second motor 19B.

Next, with further reference to FIG. 10 and FIG. 11, the operation ofthe control unit 222 of this second preferred embodiment will beexplained on the basis of the operation flowchart for the target currentallocating part 250 shown in FIG. 12.

The target current allocating part 250 reads in the target currentsignal IT from the target current setting part 231 shown in FIG. 10(step ST201) and receives the failure signal K from the failuredetecting part 242. The failure signal K is for example a 2-bit signal,with ‘00’ meaning that the first and second motors 19A, 19B are bothnormal, ‘01’ meaning that the first motor 19A is normal and the secondmotor 19B has failed, ‘10’ meaning that the first motor 19A has failedand the second motor 19B is normal, and ‘11’ meaning that the first andsecond motors 19A, 19B have both failed.

From this failure signal K it is determined whether either or both ofthe first and second motors 19A, 19B has failed (step S202). Here, it isdetermined whether or not there is a ‘1’ in either of the bits of the2-bit signal. When there is no failed motor, i.e. when the two motorsare both working normally, normal control is carried out (step S205).Here, when the two motors are the same size, normal control meanspassing the same current through both of them, and when they aredifferent sizes, normal control means allocating currents to them incorrespondence with their sizes. Further, the current may be passedthrough only one motor instead of being passed through the two motorssimultaneously.

When there is a failed motor, it is determined which motor is the failedmotor and which motor is the normal motor (step S203). Here, when thereis no normal motor, i.e. the failure signal K is ‘11’, control of theelectric power steering apparatus is stopped (step S206). When it isdetermined that there is a normal motor, i.e. when the failure signal Kis either ‘01’ or ‘10’, the allocation of current to the normal motor israised in accordance with the failure signal K (step S204).

FIG. 13 is a view showing a relationship between loads on motors and adriver. The normal ratio of the auxiliary steering forces provided bythe first motor 19A and the second motor 19B and the steering forceexerted by the driver is 5:5:1. The hatching shows the steering forceapplied by the driver. For example, when the first motor 19A has failed,by the target current allocating part 250 shown in FIG. 10 the currentsupplied from the motor driving circuit 248B to the second motor 19B isincreased and the auxiliary steering force provided by the second motor19B increases. The graph on the right in FIG. 13 shows that the burdenon the driver has been reduced by the auxiliary steering force of thesecond motor 19B. Here, the ratio of the auxiliary steering force of thesecond motor 19B to the steering force applied by the driver is about7:4.

Next, a control unit of a third preferred embodiment of the inventionwill be described, on the basis of FIG. 14 through FIG. 17.

FIG. 14 is a block diagram showing the internal construction of acontrol unit 322 of a third preferred embodiment of the invention.Elements essentially the same as elements shown in FIG. 10 of the secondpreferred embodiment have been given the same reference numerals andwill not be described again here.

The control unit 322 of this third preferred embodiment has currentdetecting parts 344A, 344B for measuring the currents flowing throughthe motor driving circuits 248A, 248B. The current detecting parts 344A,344B detect the currents from the resistance values of shunt resistancesR1, R2 and the potential differences across the shunt resistances R1,R2, and output current signals IA, IB. The detected currents flowingthrough the motor driving circuits 248A, 248B are inputted to a targetcurrent allocating part 350 as the current signals IA, IB.

FIG. 15 is a block diagram showing the internal construction of thetarget current allocating part 350. The target current allocating part350 is made up of a failed motor identifying part 351 for receiving thefailure signal K and identifying the failed motor, a current allocationdetermining part 352 for receiving the target current signal IT anddetermining the motor currents to be allocated to the two motors 19A,19B and outputting current signals SA′, SB′ accordingly, a current limitvalue setting part 353 for receiving a signal from the failed motoridentifying part 351 and setting a current limit value for the normalmotor, and a timer 354 operating on the basis of the current limit valueand the current flowing through the motor.

The failed motor identifying part 351 identifies which motor has failedon the basis of the failure signal K, and outputs a signal KM pertainingfor example to the normal motor to the current allocation determiningpart 352 and the current limit value setting part 353. When there hasbeen no failure, the failed motor identifying part 351 does not output asignal KM to the current limit value setting part 353. The current limitvalue setting part 353 receives the signal KM pertaining to the normalmotor and sets the current limit value for the normal motor to a failurecurrent limit value. This failure current limit value, when one of themotors has failed, is set larger than the normal current limit value, toincrease the auxiliary steering force provided by the normal motor. Thecurrent allocation determining part 352 receives the target currentsignal IT and the signal KM pertaining to the normal motor and a currentlimit value signal L, and determines the currents to be supplied to thefirst motor 19A and the second motor 19B. For example when the firstmotor 19A has failed and the second motor 19B is normal, it sets thecurrent to the first motor 19A to 0 and passes a current larger than thenormal current limit value through the second motor 19B incorrespondence with the failure current limit value.

Next, with further reference to FIG. 14 and FIG. 15, the operation ofthe control unit 322 of this third preferred embodiment will beexplained on the basis of the operation flowchart for the target currentallocating part 350 shown in FIG. 16.

The target current allocating part 350 receives the failure signal Kfrom the failure detecting part 242, as shown in FIG. 14. The failuresignal K is for example a 2-bit signal, with ‘00’ meaning that the firstand second motors 19A, 19B are normal, ‘01’ meaning that the first motor19A is normal and the second motor 19B has failed, ‘10’ meaning that thefirst motor 19A has failed and the second motor 19B is normal, and ‘11’meaning that the first and second motors 19A, 19B have both failed.

From this failure signal K it is determined whether either or both ofthe two motors 19A, 19B has failed (step S301). Here, it is determinedwhether or not there is a ‘1’ in either of the bits of the 2-bit signal.When there is no failed motor, i.e. when the motors are workingnormally, a count C, which will be further discussed later, is set to 0(step S309). When there is a failed motor, it is determined which motoris the failed motor and which motor is the normal motor (step S302).Here, when there is no normal motor, that is, when the failure signal Kis ‘11’, control of the electric power steering apparatus is stopped(step S308). When it is determined that there is a normal motor, i.e.when the failure signal K is ‘01’ or ‘10’, the current limit value ofthe normal motor is set to a failure current limit value (step S303).Because when the current limit value of the normal motor is set to thefailure current limit value a motor current greater than the normalcurrent limit value can be passed through the normal motor, it ischecked whether or not the current through the normal motor is greaterthan the normal current limit value (step S304). When the currentthrough the normal motor is smaller than the normal current limit value,the count C, which will be further discussed later, is set to 0 (stepS309). When the current through the normal motor is greater than thenormal current limit value, the timer 354 shown in FIG. 15 is operatedand the count C is incremented by 1 (step S305). The motor current beingabove the normal current limit value is not usually desirable, from thepoint of view of durability; however, if it is just for a predeterminedperiod, the normal current limit value can be exceeded without affectingdurability. Therefore, when there is a failure, it is possible to obtainan auxiliary steering force which cannot be obtained when the normalcurrent limit value is set, by setting the current limit value to afailure current limit value. For the reason above, the predeterminedtime for which the motor current exceeds the normal current limit valuemust be within the range of durability of the motor. So, when the countC of the timer 354 has risen above α (step S306), the current limitvalue is returned to the normal current limit value (step S307).

In the process described above, when after the timer 354 is operated oneof the motors is failed and the other motor is normal, processing passesthrough S303 again, but at this time the current limit value of thenormal motor is held at the failure current limit value, and acomparison of the current passing through the normal motor and thenormal current limit value is carried out (step S304). At this time,when the current through the normal motor is below the normal currentlimit value, because the load on the motor is not above normal, thecount C of the timer 354 is set to 0 (step S309). By this means it ispossible to operate the motor more efficiently.

FIG. 17 is a view showing the relationship between the loads on themotors and the driver. It will be assumed that the normal ratio of theauxiliary steering forces of the first motor 19A and the second motor19B and the steering force applied by the driver is 5:5:1. The hatchingshows the steering force applied by the driver. For example, when thefirst motor 19A has failed, the current limit value setting part 353 ofthe target current allocating part 350 shown in FIG. 15 sets the currentlimit value for the second motor 19B to a failure current limit value.Consequently, a current greater than the normal current limit value canflow through the second motor 19B. As a result, the auxiliary steeringforce provided by the second motor 19B increases. The graph on the rightin FIG. 17 shows that the auxiliary steering force provided by thesecond motor 19B has lightened the steering force load on the driver.And this shows that the steering force burden on the driver has beenlightened more than when simply the allocation of motor current ischanged, as shown in FIG. 13 of the second preferred embodiment. Here,the ratio of the auxiliary steering force from the second motor 19B tothe steering force from the driver is about 8:2.

As described earlier, with an electric power steering apparatusaccording to the second preferred embodiment, a plurality of motors forapplying steering forces to steering road wheels are provided, anallocating part for allocating auxiliary steering forces to the motorsis provided, and when at least one of the motors has failed, thisallocating part increases the allocation of auxiliary steering forceallocated to the normal motors. And as a result, the load on the driverat times of motor failure is lightened.

In the third preferred embodiment, because a current limit value settingpart for setting current limit values for the motors is provided, andwhen at least one of the motors has failed, the current limit valuesetting part sets the current limit value for a normal motor to afailure current limit value, a motor current greater than the normalcurrent limit value can be allocated to the normal motor so as to makefull use of the capacity of the normal motor, and the burden on thedriver can be reduced further.

Next, with reference to FIG. 18 and FIG. 19, a specific example of theinternal construction of the gearbox 24A and the power transmissionmechanism 18A will be described. FIG. 18 is a sectional view on the line18—18 in FIG. 1, and shows the relationship between the first motor 19A,the pinion shaft 12 a and the rack shaft 14. FIG. 19 is a sectional viewon the line 19—19 in FIG. 18.

In FIG. 18, in a housing 24 a forming the gearbox 24A, the pinion shaft12 a is rotatably supported by two bearings 51, 52. Inside the housing24 a are housed the first rack-and-pinion mechanism 15A and the firstpower transmission mechanism (speed reducer) 18A, and above it ismounted the steering torque detecting part 20. An upper opening of thehousing 24 a is closed by a lid 53, and the lid 53 is fixed with bolts54. The pinion gear 13A provided on the lower end of the pinion shaft 12a is positioned between the bearings 51, 52. The rack shaft 14 ispressed against the pinion gear 13A by a contact member 57 guided by arack guide 55 and urged by a compressed spring 56. The first powertransmission mechanism 18A is made up of a worm gear 59 fixed to atransmission shaft (worm shaft) 58 (see FIG. 19) connected to the outputshaft 19 a of the first motor 19A, and a worm wheel 60 fixed to thepinion shaft 12 a.

The steering torque detecting part 20 is made up of a steering torquedetection sensor 20 a disposed around the pinion shaft 12 a and anelectronic circuit part 20 b for electrically processing a detectionsignal outputted from the steering torque detection sensor 20 a. Thesteering torque detection sensor 20 a is attached to the lid 53.

FIG. 19 shows the first motor 19A and the internal construction of thecontrol unit 22.

The first motor 19A has a rotor 72 made up of permanent magnets fixed toa rotating shaft 71 and a stator 74 disposed around the rotor 72. Thestator 74 has a fixed winding 73. The rotating shaft 71 is rotatablysupported by two bearings 75, 76. One end of the rotating shaft 71 isthe output shaft 19 a of the first motor 19A. The output shaft 19 a ofthe first motor 19A is connected to the transmission shaft 58, so thatrotary power is transmitted, by a torque limiter 77. The worm gear 59 isfixed to the transmission shaft 58, as mentioned above, and the wormwheel 60 meshes with this. The above-mentioned motor angle detectingpart (position detecting part) 23A for detecting the angle (rotationalposition) of the rotor 72 of the motor 19A is provided at the other endof the rotating shaft 71.

The motor angle detecting part 23A is made up of a rotor 23 a fixed tothe rotating shaft 71 and a detecting device 23 b which uses a magneticaction to detect the angle of this rotor 23 a. For example a resolver isused as the motor angle detecting part 23A. A 3-phase a.c. motor currentis supplied to the fixed winding 73 of the stator 74. The constituentelements of the first motor 19A are housed in a motor case 78.

The control unit 22 is housed in a control box 81 mounted on the outsideof the motor case 78 of the first motor 19A. The control unit 22consists of an electronic circuit made by mounting electronic circuitcomponents on a circuit board 82. The electronic circuit componentsinclude a 1-chip microcomputer and peripheral circuitry thereof, apre-driving circuit, a FET bridge circuit and an invertor circuit. Amotor current (the drive control signal SG1A shown in FIG. 1) issupplied from the control unit 22 to the fixed winding 73 of the firstmotor 19A. And an angular position signal SG2A detected by the motorangle detecting part 23A is inputted to the control unit 22.

On the basis of this mechanical construction, the first motor 19Aoutputs a turning force (torque) for supplementing the steering torque,and this turning force is applied to the pinion shaft 12 a, i.e. thesteering shaft 12, via the first power transmission mechanism 18A.

Next, a particular construction of a fourth preferred embodiment will bedescribed, with reference to FIG. 2, FIG. 20 and FIG. 21. FIG. 20 showsthe relationship between the first and second rack-and-pinion mechanisms15A and 15B inside the first and second gearboxes 24A, 24B on the rackshaft 14 (shown in vertical section in the figure and with its centralpart truncated). FIGS. 21A and 21B show two motor torque fluctuationcharacteristics (a), (b) corresponding to the first and secondrack-and-pinion mechanisms 15A and 15B, and FIG. 21C shows these motortorque fluctuation characteristics (c) combined.

As shown in FIG. 2, the gearboxes 24A, 24B are disposed in left andright locations in the axial direction of the rack shaft 14. The firstgearbox 24A is a gearbox connecting to the pinion shaft 12 a at thebottom of the steering shaft 12. The second gearbox 24B is a gearbox forincorporating the second motor 19B into the two-motor electric powersteering apparatus 10 (see FIG. 1).

The first and second motors 19A, 19B are connected to the first andsecond first gearboxes 24A, 24B by the first and second powertransmission mechanisms 18A, 18B. As shown in FIG. 1, auxiliary steeringtorques are applied by the rotational driving forces of the two motors19A, 19B to the rack shaft 14, which has steering road wheels at itsends (the front wheels 17, 17). The relationship between the piniongears and the rack gears in the two gearboxes 24A, 24B on the rack shaft14 is as shown in FIG. 20.

In FIG. 20, first and second rack gears 14 a, 14 b are formed on therack shaft 14 in locations corresponding to the two gearboxes 24A, 24B.The first and second pinion gears 13 a, 13 b mesh with these two rackgears 14 a, 14 b. The first pinion gear 13 a is the pinion gear fixed tothe pinion shaft 12 a shown in FIG. 1. The second pinion gear 13 b isthe pinion gear fixed to the pinion shaft 12 b (see FIG. 1) providedinside the second gearbox 24B.

As shown in FIG. 20, in the meshing relationships of the two rack gears14 a, 14 b formed on the rack shaft 14 and the first and second piniongears 13 a, 13 b corresponding to these rack gears, the first and secondpinion gears 13 a, 13 b are mounted with their angular positionsessentially staggered by 180° in phase.

Specifically, in this example, as shown in FIG. 20, the positionalrelationship is so set that when a tooth bottom of the first pinion gear13 a is positioned facing a tooth tip of the first rack gear 14 a, atooth tip of the second pinion gear 13 b is positioned facing a toothbottom of the second rack gear 14 b. This meshing relationship of thegear teeth is an example, and as will be discussed in the following itmay be set in any way such that the motor torque fluctuationsoriginating in torque ripple arising in the first and second motorsbecome reverse in phase (staggered by 180°) so that the motor torquefluctuations cancel out in the rack shaft 14.

When the positional relationships of the rack gears 14 a, 14 b on therack shaft 14 and the pinion gears 13 a, 13 b are pre-set as describedabove, the characteristics 91, 92 shown in FIGS. 21A and 21B arise.

In FIGS. 21A and 21B, the horizontal axis shows angle and the verticalaxis shows torque fluctuation. Accordingly, when the two motors 19A, 19Boperate and power is transmitted to the rack shaft 14 by way of the twopinion gears 13 a, 13 b and the two rack gears 14 a, 14 b, in thetransmission torque in the first gearbox 24A the fluctuationcharacteristic 91 arises, and in the transmission torque in the secondgearbox 24B the fluctuation characteristic 92 arises. Because thepositional relationship of the first and second pinion gears 13 a, 13 bwas set as described above, in the waveform of the fluctuationcharacteristic 91 and the waveform of the fluctuation characteristic 92the phases are completely opposite. Consequently, as shown in FIG. 21C,in the rack shaft 14 as a whole, as the torque transmitted, thefluctuation characteristics 91 and 92, which are opposite in phase,cancel each other out, the torque ripples are converged, and asuppressed torque fluctuation characteristic 93 can be obtained. As aresult, it is possible to produce an auxiliary steering torque having novibration.

With this fourth preferred embodiment, because in an electric powersteering apparatus two motors having the same capabilities are used togenerate an auxiliary steering torque, by staggering through 180° andthereby making opposite in phase between the two motors the phaserelationships of the pinion gears and racks in the rack-and-pinionmechanisms inside the gearboxes for the respective motors, the torqueripples of the motors can be canceled out on the rack shaft 14 andvibration and noise can be eliminated.

Although in the fourth preferred embodiment described above the phasesrelating to the two motors 19A, 19B in the rack-and-pinion mechanisms inthe first and second first gearboxes 24A, 24B were staggered by 180°,the invention is not limited to this, and alternatively for example thephases of the motors may be staggered by 180° by adjusting therelationships of the rotors and stators in the motors.

Although in the fourth preferred embodiment an electric power steeringapparatus which is a mechanical transmission mechanism incorporating aspeed reducer was described by way of an example, the invention is notlimited to this, and may also for example be applied to a steer-by-wiresystem from which a mechanical linkage has been eliminated.

As described above, with this fourth preferred embodiment of theinvention, because in a steering apparatus having two motors forassisting with steering the phases of the two motors are staggered by180°, motor torque fluctuations originating in torque ripples arising inthe two motors can be suppressed, vibration in the steering system canbe reduced, the steering feel can be improved, and controllability canbe raised. And, the need for the torque fluctuations of each individualmotor to be kept small is eliminated, and the outputs of the motors canbe raised at low cost.

Obviously, various minor changes and modifications of the presentinvention are possible in the light of the above teaching. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

The present disclosure relates to the subject matters of Japanese PatentApplications, No. 2002-188003 filed Jun. 27, 2002, No. 2002-239737 filedAug. 20, 2002, and No. 2002-290812 filed Oct. 3, 2002, the disclosuresof which are expressly incorporated herein by reference in theirentireties.

1. A motor driving method for a steering apparatus, comprising the stepsof: providing a steering system including a rack shaft and two motorsdisposed in spaced relation to one another for applying an auxiliarysteering force to different positions on the rack shaft of the steeringsystem; in operating the two motors, operating one of the two motorsfirst; and after operating the first motor, operating the other motor,wherein when only one of the two motors is being operated, motor failuredetection for said only one motor is carried out.
 2. A motor drivingmethod according to claim 1, wherein in the step of operating one of themotors first, the same motor is always used.
 3. A motor driving methodaccording to claim 1, wherein the output capacities of the two motorsare different.
 4. A motor driving method for a steering apparatus,comprising the steps of: providing two motors for applying an auxiliarysteering force to a steering system; in operating the two motors,operating one of the two motors first; and after operating the firstmotor, operating the other motor, wherein in the step of operating oneof the motors first, the two motors are used alternately.
 5. A motordriving method according claim 4, wherein the output capacities of thetwo motors are different.
 6. A motor driving method according to claim4, wherein when only one of the two motors is being operated, motorfailure detection for said only one motor is carried out.
 7. A motordriving method for a steering apparatus, comprising the steps of:providing two motors for applying an auxiliary steering force to asteering system; in operating the two motors, operating one of the twomotors first; and after operating the first motor, operating the othermotor, wherein in the step of operating one of the motors first, thesame motor is always used, and wherein when the steering direction isone direction a first of the motors is operated first and then thesecond motor is operated, and when the steering direction is the otherdirection the second motor is operated first and then the first motor isoperated.
 8. A motor driving method according to 7, wherein the outputcapacities of the two motors are different.
 9. A motor driving methodaccording to 7, wherein when only one of the two motors is beingoperated, motor failure detection for said only one motor is carriedout.